This invention relates to electron beam pattern generation systems used for forming a plurality of semiconductor device chips on a semiconductor wafer.
In the manufacture of a semiconductor device, use has been made of an electron beam pattern generation system for forming, for instance, an impurity diffusion mask pattern on a chip by using a resist. More particularly, the electron beam pattern generation system is used for drawing an exposure pattern on a resist coating on a semiconductor wafer with an electron beam emitted from an electron beam source by causing deflection of the electron beam according to pattern data. The exposure pattern is a dot pattern consisting of white and black dots. When a positive type resist is used, the white dots are formed in positions which are struck by the electron beam and the black dots in positions which are not struck by the beam. For on-off controlling the electron beam in accordance with the white and black dots, the pattern data is converted into digital or bit data consisting of bits of "1"s and "0"s before being supplied to the electron beam pattern generation system. The digital pattern data or bit pattern data is supplied from an external data source to a temporary storage unit and stored therein, and then it is read out and transferred to the electron beam pattern generation system under the control of an electronic computer. However, the bit pattern data consists of an enormous quantity of bits. For example, where a pattern with a bit density of 1 micron per bit is formed on an 8 mm.times.8 mm chip, a storage capacity of about 8 megabytes is required, and the storage capacity has to be increased to 32 and 128 megabytes when the bit density is increased to 0.5 and 0.25 micron per bit respectively. Usually, a plurality of chips are formed on a circular wafer, for instance with a diameter of 15 cm, and an extremely great storage capacity is necessary to be able to store digital pattern data for obtaining the exposure of the entire wafer with an electron beam.
The drawing speed of the electron beam pattern generation system is increasing year after year, whereas the speed of reading out pattern data from the storage unit is not keeping pace with the drawing speed increase. Therefore, limitations are imposed upon the upper limit of the speed of operation of the electron beam pattern generation system.